Montana Tracy ; What are the CC you have now with the new head/piston kit? I was talking to a friend last weekend out in Yermo that mentioned the head/piston overbore kit could bump it 25cc or more.

Searaider

Hmmmmmm... an increase in the compression ratio of 10% and displacement of 2.2%. Nominally, the torque is up about 12 percent. I guess you'd feel that in normal trail riding. I tend to lug on rough trails.

I wonder if this will mean anything on the mid-top end without modifying the head and valves.

I guess I'm also wondering why they just didn't go to 250cc. Is there some problem with the jug? An increase to 250cc (or even 240cc), combined with the 10.5 to 1 compression, would be a nominal 20-22% increase in torque.

When I was considering the engine build on my Sportster I asked if anyone just raised the compression. The answer was "rarely, but why do that?". Since it is almost trivial to go to 1200cc that's the basic path. Mine went from 883cc 9-1 CR to 1200cc 10.4-1 CR. Nominally a 66% increase in torque! However, to exploit this at RPM's above 4,500 the heads needed work - larger valves and flowed - along with cams. Since a Sportster uses a cam for every valve (4 of them) that's not cheap. The head work and cams was good for getting 10HP at higher RPM but cost $1500 of the $2500 build cost. That is, without the heads/cams the engine would not be breathing well at higher RPM's but would be pretty stout in the low-mid range. A very popular and cheap DIY 1200cc conversion is to go to 9.5-1 CR and no head or cam changes. Increase in torque is about 38%. These dyno out at 60-65HP and are sweet in everyday riding.

[ July 15, 2006: Message edited by: Randy ]

Bumping up the little XT engine is an intriguing idea because who doesn't want more power?...

...but how much does raising the displacement and compression increase the heat generated by the engine? Are the cooling fins sufficient to dissipate this increased heat? And does higher compression force you to use more expensive higher octane gasoline to overcome the increased tendency to detonation? Are timing changes made to fit the new combustion properties of the engine?

The newer XT engines also have ceramic plating on the cylinder walls that is said to have excellent heat dissipation and wear properties. Is this replaced when the engine is bored? Is the bottom end strong enough to withstand the higher pressures exerted on it.

It's possible that the boring necessary to increase to 250cc brings the heat of the cylinder walls too close to the oil passages

The bottom line is that I'm wondering how much reliability and longevity is traded off for increased power?


Greg

[ July 15, 2006: Message edited by: Greg ]

well the kit will come with a cam if you want one but the kit doesnt require it, and yes greg you would need to run premium gas.

i asked the same questions as you guys about a 250cc kit and was basically told that it cant work. there is not enough room to allow it. mind you this wasnt from powroll but from a yamaha mechanic. now powroll told me that there has to be enough interest in a product to produce it. makes sense to me.


i guess im going to be the test pig. so i will post up my results with gas mileage, driveability, and most important power increase. as some of you have seen i already have a baseline of what my bike puts down at the back tire (hp wise). im hoping once its all done i will have my goal of around 22ish hp. then i will be completeing my motard conversion.

-shaun h

[ July 15, 2006: Message edited by: awdspider ]

I can answer some of Greg's questions
through my experiece with my engine work which is nearly identical to Powroll's kit.

I have had no over heating problems. Timing was not an issue although carb jetting will need to be changed with the different cam. I have the same piston through Wiseco and I have not had to use premium fuel yet to avoid predetonation, but it is a distinct possibility.

The four-over bore (71MM)only adds about 5ccs so if the bike's bottom end is sound to start with there should be no problem with added stress. In 800 miles with my bike since my engine work there have been no problems.

Boring the cylinder will eliminate the ceramic coating. The cylinder can be recoated but it is quite expensive. My bike is an older bike so the cylinder wasn't coated anyway.

In an 80 mile jaunt today my bike averaged 69mpg. Not bad considering the way my bike is geared.

[ July 15, 2006: Message edited by: Montana Tracy ]

MT,

if you had to guess, about how much of an increase in power would you say you gained. i would think that with your mods your little xt might not be so little anymore.


shaun h

Shaun, It is so hard to be objective and to make a guess, but I'd hazzard a guess of 25 - 30%.

The TM33 pumper carb conversion is the most significant mod I've done to the engine. The throttle response is outstanding. No loping and no stumble, ever, at any power range or throttle position. Plus, easy start and quick warmup.

The bore is in direct relationship to the stroke. If the bore gets to large for the stoke you have an over square situation. The bore becomes greater then the stroke. When it gets out of square it causes mechanical problems and you get diminishing returns in relation to its increase in power.

Bumping the compression to high will cause the engine to run warmer. Any increase in power will create additional heat. This doesn't mean that the heat will be too high if you do what Tracy has done. But it will increase, all things being equal. Higher compression has a tendency towards pre-ignition as well and quite often requires a higher octane gasoline.

Even the muffler and air box mods have an effect. It is minimal but it is there.

More power equals more heat and more wear in the same size engine.

Cigar Mike

I've had experience with 2 big bore kits my first was with a 305cc kit I had installed on a honda tl 250 that i converted into an enduro mount never had a bit of problem with the kit and it made the TL into a torque monster. the second was with a honda xr250l that i had installed a 280cckit . the xr250l's have a tendency to run hot any way ( no oil cooler on the xrl's) after I got the bike back it ran considerably hotter because the larger displacement and higher compression piston . It never let me down I just changed the engine oil more often and put the best oil money could buy at the time in it( oil is cheap engines are'nt) Powroll also re-strokes the engines one can also obtain a larger displacement by changing the stroke. A lot of their kits are available as a combination larger bore and dif. stroke.I'm sure if this kit becomes a good seller powroll will them offer a stroker kit for the xt.any way I want tell to those considering the larger bore kits that the 2 that so far i've had experience with were very reliable and trouble free once you get the re-jetting dialed in you got it made.

[ July 15, 2006: Message edited by: dirtbikenut ]

dirtbikenut

I hope there was no misunderstanding of my post. My post is related to how much over you can go. They are not going to punch the 225 out to 400cc's. My point is there is a limit and there are reasons why.

It is a fact that more power creates more heat and that causes more wear. How much heat and how much wear is another question.

I have overbored bikes myself and it has not reduced the reliabliy. But it didn't mean that I wasn't wearing it out faster.

There's no free lunch.

Cigar Mike

[ July 15, 2006: Message edited by: Cigar Mike ]

[ July 15, 2006: Message edited by: Cigar Mike ]